In all industrial facilities where steam is used as an energy source, it has become a typical practice to avoid exhausting of steam to the atmosphere and to collect all such exhaust steam in a closed system of piping, which is maintained at some pressure greater than atmospheric pressure.
The purpose of such a system is essentially for condensation of the exhausted steam as distilled water, which contains appreciable sensible heat for use as preheated boiler feed water. This avoids the depositing of scale within the boiler or steam generator. Also, this system of condensation avoids the presence of steam plumes in the atmosphere. The pressure within the collecting system is kept as low as possible in order to insure maximal use of the high pressure steam energy, and is typically in the range of 10 to 25 psi gauge, according to the initial steam pressure prior to exhaust. A typical ratio of high pressure and low pressure is in the order of 10 to 1.
Because of the operation as outlined, the value of exhaust steam per 1,000 pounds weight is not great, whereas steam at 100 psi gauge costs a minimum of $1.50 per 1,000 pounds, or substantially 10 times as much. Because of demand for conservation of heat energy there is a frantic search for greater use of exhaust steam values.
Because the state of the art of smoke suppression in flame burning of hydrocarbons requires steam at great hourly rates, there is a continued look to exhaust steam for possible use in flares for smoke suppression. However, because of the low pressure typical of exhaust steam, such low pressure steam is rarely used for smoke suppression in flares in the conventional manner.
This invention permits the use of low pressure exhaust steam for smoke suppression at the flare, for either complete suppression of smoke, or for a sharp reduction in demand for high pressure steam, according to the particular nature of the hydrocarbons being flared. It also provides for premixture of steam with hydrocarbons, prior to burning, in such manner as to insure the presence of vapor phase water, which is homogeneously mixed with vapor phase hydrocarbons, prior to burning, where a significant mol percentage of the mixture is as water vapor.
It is now common in the art of smoke suppression to inject steam into the hydrocarbons after burning of the hydrocarbons has started. Steam injected from high pressure, from orifices, is moving at critical or sonic velocity, and for that reason is a source of kinetic energy which is spent in air entrainment with the steam and in creation of turbulence, as the steam-air mixture enters the burning hydrocarbons. Injected air plus turbulence greatly enhances burning of the hydrocarbons, but a very significant result of steam injection to burning hydrocarbons is the typical reforming chemistry in the heated zone of burning hydrocarbons, according to the following equation: EQU CH.sub.4 + H.sub.2 O = CO + 3H.sub.2
it is to be noted that the reaction as shown, which alone is effective to a degree in suppression of smoke, is a vapor phase reaction which demands that water present in the combustion zone must be in vapor phase rather than as liquid, in order to accomplish greatest suppression of smoke. Smoke is suppressed because through the reaction as shown, carbon is combined with oxygen to form carbon monoxide, which is both invisible and rapid burning. On the other hand, smoke results because of the presence of free carbon as it escapes from the combustion zone.
If a small quantity of steam or water vapor is added to hydrocarbons at 60.degree.F the water vapor promptly condenses and becomes liquid particles, since the ability of hydrocarbon gases to retain water in vapor phase is governed by the hydrocarbon temperature. For example, at 60.degree.F the mol percentage of water vapor is approximately 1.75%, however, at 150.degree.F approximately 26% of the volume of a gas-water vapor mixture is a water vapor. Since the gaseous phase reaction is governed by the partial pressure of the reactants, it is obvious that for a satisfactory state of smoke suppression, the partial pressure (mol percentage) of water vapor must be a significant portion of the total pressure, and temperature elevation is required to contain this large mol percentage of water vapor where the temperature of the steam-hydrocarbon mixture is elevated suitably through the heat content of the steam added to hydrocarbon, to cause the mol-percentage of water vapor in the water vapor-hydrocarbon mixture to be significant following thorough mixture of the two gases. Also, the gas-water vapor mixture must be homogeneous for optimum reaction and smoke suppression. Furthermore, and because the source pressure for venting hydrocarbons to atmospheric pressure may be severely limited, the steam injection must not add significantly to the pressure drop from the source to the atmosphere.
There are established conditions for optimum use of exhaust steam, or low pressure steam, injection to hydrocarbon gases for the purpose of smoke suppression as the gases are flare burned, and where there is premixture of steam with hydrocarbons, as the hydrocarbons are en route to the flare for burning. Note that prior effort for such use of steam has met with limited success, and such use of steam is largely abandoned because of failure to meet the conditions as outlined above.